WATER PERCOLATION ACROSS THE SOIL-BEDROCK INTERFACE IN MOUNTAINOUS TERRAIN

Various factors influence the movement of water into bedrock, including the thickness and nature of the soil cover (e.g., pipes and barriers), the type of climate and vegetation, and the type of bedrock and the nature of its fractures. In mountains slope steepness and aspect, and its interaction with vegetation and soil development, also play a strong role, affecting percolation rates by adjusting moisture fluxes to and from the atmosphere and the down-slope movement of moisture in the soil above the bedrock. We use both observed and hypothetical data and conceptual models, and mathematical modeling of the atmospheric boundary condition and moisture movement in the soil and bedrock, to explore sensitivity of percolation to these many factors. Historically, hillslope studies at this scale have ignored percolation, or treated it as a secondary or tertiary issue, while this study features it front-and-center. Two primary controls on percolation are bedrock permeability and the availability of water at the soil-bedrock interface. Water availability is determined by slope steepness and aspect, due to its affect on atmospheric boundary conditions, and climate. Bedrock topography and slope steepness are much less important to the physics of flow, and therefore to percolation, than soil structure and bedrock type and fracturing. Varying sensitivity of percolation to various factors, coupled with heterogeneous landscape, geology, and atmospheric fluxes, leads to a complex pattern of bedrock recharge in mountainous terrain.